Air pollution due to solid fuel burning has become critical in many parts of the country. Certain ski areas have serious air pollution problems because their locations in valleys or bowls in the mountain prevent natural ventilation. The air pollution is generated by both fireplaces and wood stoves. Fireplaces are usually located in the accommodations for skiers and are burned for aesthetic reasons; wood stoves are usually located in the homes of the local population and are burned to provide heat. Air pollution has become so bad in some areas that fireplace burning has been restricted and limitations have been placed on new fireplace construction.
Pollution is caused because fireplaces and wood stoves do not burn all of the fuel vapor escaping from the burning wood and these fuel vapors are transported up the chimney into the atmosphere. For clean combustion, thorough mixing of the air and fuel vapor is required to achieve intimate contact between oxygen and fuel molecules. In addition, the mixture must be at a temperature high enough to cause the reaction to occur and sufficient time must be available to allow the reaction to go to completion. The reaction rate increases exponentially with increasing temperature so it is highly desirable to have high temperatures to complete the reactions in the shortest time possible before the reactants have escaped from the hot combustion zone of the fire and have been cooled below their ignition temperature by dilution with excess air or heat transfer to the surroundings.
Existing fireplaces and stoves use natural draft to bring oxygen to the burning surfaces of the wood as well as to cause mixing of the fuel vapor and oxygen in the combustion zone. Natural draft forces result from the buoyancy of heated air or combustion gas, bringing new air to the combustion zone. Unfortunately, these buoyant forces are extremely weak (normally less than 0.01 inches of water gage (IWG)) and result in very low velocities and turbulence levels over the burning surface. These low velocities limit the rate at which combustion can occur on the burning surface of the wood and also limit the mixing of the volatile fuel vapor with the combustion air. Because of the limited combustion rate on the wood surface and limited combustion of the volatiles in the combustion zone, temperatures in the combustion zone are usually too low to allow complete combustion of the fuel vapors before they escape from the combustion zone and cooled still further.
A further problem arises because of the coupling of the fire with the natural draft. Wood in a fireplace or stove does not burn with uniform intensity over its entire surface but rather has zones where it burns with high intensity and zones where very little combustion occurs. The zones with high intensity create the most buoyant forces and hence, the most intense local draft. The hottest part of the fire tends to become even hotter. Unfortunately, the cooler part of the fire, which badly needs a stronger draft to burn hotter and cleaner, is unable to generate that draft so it tends to cool further and pollute more. Near the end of a fireplace fire, when the hottest zones of the fire have all burned out, little draft is left to burn the smoldering logs yet remaining and to prevent large quantities of unburned volatiles from escaping into the atmosphere.
Measurements taken in an open fireplace indicate that the combustion zone in which adequate combustion temperatures may be achieved is localized to within a few inches of the burning wood and often is found only in the interstices between logs where the radiated heat from the burning surface is retained in a cavity. Fireplaces burn with large amounts of excess air (2000% excess air was measured in one test) that dilute and cool the combustion gas below combustion temperature shortly after it leaves the surface of the wood. Temperatures measured twelve inches above the fire in a fireplace are approximately 200.degree. F., well below the nominal 1000.degree. F. needed to initiate the combustion reactions. Hence, for fireplaces, the only opportunity to achieve clean combustion by burning the volatile vapors exists while the vapors are in the combustion zone, immediately adjacent to the surface of the wood.
Airtight wood stoves do not have the problem of large amounts of excess air cooling the combustion gas, as do fireplaces; however, they do transfer significant amounts of heat from the combustion zone of the fire directly to the cooler walls of the firebox. They also operate with natural draft forces and are subject to the same difficulties of low combustion rates on the surface of the wood, poor mixing of the volatiles with the air and non-uniform combustion conditions, all of which reduces temperatures in the combustion zone and limits their ability to burn the fuel vapors completely. Wood stoves have the additional problem that the combustion air is limited in order to control the combustion rate of the stove and heat losses up the chimney. Limited air means that the ability of the fuel vapor molecule to find an oxygen molecule is also limited, further preventing complete combustion of the fuel vapor. This situation is exacerbated when the airflow of a stove is significantly reduced to produce a slow burn, at which times the stove becomes a much greater source of pollution.
One approach to improved combustion is found in Maasberg (U.S. Pat. No. 3,269,383) which teaches a fireplace grate comprised of two perforate andiron tubes used to support the ends of logs in a fireplace. He provides a forced draft adjacent the ends of the logs to facilitate the initial kindling of the log fire and, if desired, to maintain combustion of the logs. Maasberg does not teach the provision of a uniform supply of forced air into the entire lower surface of the burning logs or the maintenance of a combustion zone provided by a bed of hot coals adjacent to the lower surface of the burning logs.
Similarly, Alexander and Haynes, in French Pat. No. 2,398,265, teach a grate with hollow bars, the central ones of which have orifices to produce jets of air to provide augmented combustion near the center of the fire, in the manner of a blacksmiths forge. They do not teach the provision of a uniform supply of forced air across the entire lower surface of the burning logs. In fact, their design only serves to intensify the burning rate at the center of the fire, already hot, while providing no augmentation near the periphery of the fire which is the coolest and pollutes greater. In addition, Alexander and Haynes do not teach any provision for maintaining a combustion zone of hot coals in close proximity to the bottom surface of the burning logs.